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Investment Securities (Fair Value Measurements) (Details) (Details) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Percentage of investments recorded at fair value | 89.00% | 94.00%

X
- Definition Represents the percentage of the total investment portfolio recorded at fair value based upon quoted market prices or other observable inputs. + References No definition available. + Details
Name: | wtm_PercentageOfInvestmentsRecordedAtFairValue
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration